This invention relates to an imaging device with automatic focusing function used in a video camera, etc., and more particularly to an imaging device capable of carrying out an automatic focusing with a stable detection accuracy and at a high response speed irrespective of the distance from an in-focus position of a principal object, the intensity of a secondary object, and the kind of illumination at the place where an object is positioned, etc.
FIG. 1 shows a schematic block diagram of a conventional automatic focusing imaging device using the hill-climbing method. This device is provided with an optical system 3 comprised of an afocal system lens 1 and a focusing lens 2, thus to carry out focusing by moving the focusing lens 2 in an optical axis direction.
Further, an image pick-up light from an object incident through the optical system 3 undergoes photoelectric conversion at an image pick-up element 4 such as a CCD (Charge Coupled Device), etc. to provide an electric video signal S.sub.v.
Then, this video signal S.sub.v is amplified at an amplifier 5 and is delivered to a recording circuit (not shown), and is also delivered to a band-pass filter (BPF) 6.
This band-pass filter 6 extracts a high frequency component at the central portion of a pictorial image from the video signal S.sub.v to deliver it to a detector (DET) 7, at which a focal point voltage E corresponding to the high frequency component of the video signal S.sub.v is taken out.
This focal point voltage E corresponds to sharpness of a reproduced pictorial image of the video signal S.sub.v. When the focusing lens 2 is at the in-focus position P, the focal point voltage E takes a maximum focal point voltage E.sub.max.
The focal point voltage E is then digitalized at an A/D converter 8, and focal point voltage thus digitalized is delivered to a calculation processing circuit 9.
This calculation processing circuit 9 is of a structure to take thereinto the digitalized focal point voltage E every each field to generate a control signal on the basis of this data to operate drive means 10 by this control signal to move the focusing lens 2 so that it is placed at the in-focus position.
In the calculation processing circuit 9, calculation is performed in accordance with the following equation showing a focal point voltage at an in-focus position: ##EQU1## where b is a coefficient equal to E.sub.max, .DELTA.P is a distance up to an in-focus position, and E(x) is a focal point voltage at an arbitrary time point.
Further, calculation for determining distance .DELTA.P up to the in-focus position is performed in accordance with the following equation: ##EQU2## where a is a coefficient for determining the value of the focal point voltage E, and is a quantity of movement for one field time period of the focusing lens. Further, the value of a is determined as a function of a focal length of the lens, a frequency fc (frequency of the filter for extracting a high frequency component) of an object, and a F-value of the lens, etc. and is expressed by the following equation: ##EQU3## where K is a value peculiar to the lens.
Information relating to the above-mentioned fc, K and F are delivered to the calculation processing circuit 9, at which the value of a is determined.
Namely, the calculation processing circuit is of a structure to calculate logarithm of a value obtained by dividing a focal point voltage E(x) at an arbitrary time point by a focal point voltage E(x+m) after one field time period has passed to immediately determine a distance .DELTA.P up to the in-focus position of the focusing lens 2 at this time, thus making it possible to predict an in-focus position (P) of the climbing curve from a foot position (x+m) of the mountain as shown in FIG. 2(a).
Meanwhile, in accordance with the previously described automatic focusing device, a focal point voltage from the detector 7 is quantized by the A/D converter 8. This input/output characteristic becomes a step characteristic having equidistant quantization step widths as shown in FIG. 2(b), for example.
This step signal undergoes logarithmic conversion in the calculation processing circuit 9 in order to determine the above-described .DELTA.P. Thus, the characteristic of the output versus the input of the A/D converter becomes a characteristic as shown in FIG. 2(c).
As shown in FIG. 2(c), particularly according as the level lowers, the step width becomes broad.
This means that, in the climbing voltage curve of FIG. 2(a), according the position becomes close to the foot of the mountain, the step width of the quantization level becomes broad, so the detection accuracy becomes poor.
Accordingly, since an approach is employed to predict the summit (in-focus position) of the climbing curve to carry out automatic focusing, the focal point speed can be advantageously determined, but there was the problem that the focusing accuracy becomes poor in the case of predicting the summit (in-focus position) in the vicinity of the foot of the mountain.
Further, in the conventional other automatic focusing device, in the case of picking up an image of an object, detection of a focus is carried out by a focus detection window 12 provided at the central portion of an imaging frame 11 as shown in FIG. 3A.
Accordingly, in the case where a principal object 13 deviates from the focus detection window 12 as shown in FIG. 3B, there was the problem that automatic focusing is erroneously operated.
Further, in order to prevent such an erroneous operation, as disclosed in the Japanese Patent Application Laid Open No. 126976/85, for example, two large and small focus detection windows are provided at the central portion of a screen whereby when a principal object deviates from the small focus detection window, so the focus voltage lowers, the small focus detection window is switched to the large focus detection window to carry out auto focusing by the large focus detection window if the focus voltage is above a prescribed voltage.
However, also in this case, only a principal object is not necessarily included in the large focus detection window. If the contrast of any other object is higher than that of the principal object, automatic focusing was disadvantageously erroneously operated.
Furthermore, there is an automatic focusing imaging device different from the above-described two conventional devices.
Hitherto, there is known an automatic focusing device to take out, as a focal point voltage, a quantity of high frequency band components of a video signal obtained by picking up an image of an object to move a focusing member such as an imaging element or a focusing lens, etc. so that the focal point voltage becomes maximum.
However, this auto focusing device has the problem that, e.g., at the time of picking up an image under a fluorescent lamp, the focusing operation becomes unstable by the flicker component.
The ON/OFF period of a fluorescent lamp lighted, e.g., at a power supply frequency of 50 Hz is 1/100 seconds. An image picked up, by a camera having a vertical scanning frequency of 60 Hz, an image illuminated by this fluorescent lamp corresponds to the case where a signal of 100 Hz is sampled at 60 Hz. In this case, a component of 20 Hz is generated.
This component is so called "fluorescent lamp flicker".
To improve this, as disclosed in the Japanese Patent Application Laid Open No. 59274/88, for example, there is proposed an automatic focusing device to extract a luminance signal from a video signal to divide the above-described high frequency band component by this luminance component to thereby provide a focal point voltage from which the flicker component is removed to allow the device to carry out focusing on the basis of this focal point voltage, thereby permitting a stable focusing operation free from bad influence by the flicker.
In the conventional focusing device, since focus is generally detected at the central portion of the image pick-up frame, a person who picks up an image positions a principal object at the central portion within the image pick-up frame to pick up an image, thereby allow the principal object to be in focus. However, in the case where a principal object deviates from the central portion of the image pick-up frame, the automatic focusing device may be erroneously operated. To prevent this, there is proposed such an automatic focusing device to arbitrarily move the measurement window within the image pick-up frame so that the high frequency band component becomes large, or to divide the area within the image pick-up frame into a plurality of blocks to determine an optimum focusing position (the position where the principal object is in focus) of the entirety of the image pick-up frame on the basis of focus information of respective blocks (see Japanese Patent Application Laid Open No. 284181/89).
In the above-mentioned automatic focusing device adapted to carry out the focusing operation on the basis of information of the high frequency band component taken out from the distance measurement window changeable within the image pick-up frame, if the means for eliminating the influence by the flicker as disclosed in the above-described Japanese Patent Application Laid Open No. 59274/88, the problem as described below would occur.
Namely, while the high frequency band component is taken out from the distance measurement window which is only a portion of the image pick-up frame changeable in dependency upon movement of the principal object, the luminance component corresponding to the entirety of the image pick-up frame is taken out. An employment of such an approach results in the fact that the high frequency band component taken out from the distance measurement window is divided by the luminance component taken out from the entirety of the image pick-up frame to thereby eliminate the flicker component. Thus, an optimum flicker elimination corresponding to the changeable distance measurement window cannot be carried out, so the focusing operation would be erroneously operated.
Further detailed discussion will be conducted in connection with this. For example, in the case where an image is picked up within a room under a fluorescent lamp where a light having no flicker component like a sun light is admitted from a window, let consider within the same screen the case where a principal object is present under the fluorescent lamp and the case where a principal object is present near the window. The distance measurement window is positioned both at the portion below the fluorescent lamp and the portion near the window in correspondence with the principal object. A high frequency component is taken out from the distance measurement window to carry out focusing operation in order to allow the principal object to be in focus. In this focusing operation, although the manner of occurrence of flickers vary in dependency upon whether the principal object is present at the position below the fluorescent lamp or at the position near the window, since a flicker component of the high frequency component is eliminated by using a luminance signal taken out from the entirety of the image pick-up frame, the processing for eliminating the same quantity of flickers is carried out. As a result, an optimum flicker elimination corresponding to respective positions of the changeable distance measurement window cannot be conducted, so the focusing operation would be erroneously operated.